Studies are underway to determine the degree to which hygroscopic growth of particles as they are inhaled affects their deposition in the respiratory system. This is being studied several ways -- through the use of models, animals and human subjects. This approach is expected to provide data which are realistic in the sense of applicability to human studies. The model and animals studies will provide regional scale detail of particle deposition. We are also measuring the extent of neutralization of inhaled acid sulfate aerosols by respiratory NH3 prior to deposition. The production of NH3 in the mouth is being related to specific bacterial organisms. The final part of this objective is to identify populations which, because of low levels of NH3 in the upper airways, may be at increased risk if exposed to acid sulfates. The third objective is the study of the functional responses of allergic adolescents to acid aerosols present in community air. The response to these aerosols is being studied both at rest and during exercise.